Measurement of the radiometric spectral reflectance of one or more arbitrary areas on a sample can be used to characterize the physical properties of the sample. Such a spectrally resolved measurement is often reduced to a photometric color in a three dimensional (e.g. RGB, Lab, HSL, etc.) space which expresses the appearance to a human observer. Color is a human perception and affects economic value in many applications. For example, the color of meat and distribution of the marbling, the color of fruits and vegetables, the color of manufactured goods and processed foods, the colors of materials used in clothing, upholstery and other industrial processes. What is needed is a rapid, repeatable and accurate method of measuring colors, on multiple arbitrary areas on a product. This would be a critical tool to ensure the product's quality and, therefore, its economic value.
Among the many standards for measurement of color, the most widely accepted are the ones developed by the CIE (International Commission on Illumination); and these have been adopted in many industries. Measurement of color is most commonly achieved by using a contact spectrophotometer which uses an integrating sphere for accurate radiometric measurement of the spectral reflectance and its subsequent conversion to photometric units. Although accurate, the measurements are typically made over an area of only a few millimeters and do not have the spatial resolution needed to measure the color patterns on many natural and manufactured surfaces.
Absolute color measurements are typically computed and communicated using international standards such as CIE 1931 Lab space. This classic standard is described in “The C.I.E. colorimetric standards and their use” in Transactions of the Optical Society, 33(3):73 December 1930 by T Smith and J Guild.
One aspect of precision color measurement has been to characterize a sample using one or more standardized reference illuminants. For instance, U.S. Pat. No. 8,064,057 describes approximating multiple reference illuminants by using a basic lamp plus LEDs (light emitting diodes) in different combinations to compensate for the differences between the basic lamp and the reference illuminant(s).
Another aspect of accurate color measurement has been to characterize a sample using a combination of a spectrophotometer and a camera. For example, U.S. Pat. No. 7,230,707 describes color measuring using a spectrophotometer in combination with a camera.